This invention relates to an image magnification control device for a camera, wherein a photographic lens for a camera including single lens reflex cameras and video still cameras is driven and controlled by a zoom drive means so as to automatically control an image magnification of the photographic lens to a predetermined magnification.
A desireable feature for a power zoom camera is to be able to maintain a constant image magnification of an object as the distance between the object and the camera varies. For instance, when taking a series of continuous photographs, of for instance, a baseball player running to catch a ball, the photohgrapher may wish that the baseball player occupies the same area of the film frame as the player moves. This can be accomplished by varying the focal length of the lens the player moves. Unfortunately, as individual can not change the magnification value (by changing the focal length of the lens) fast enough and accurately enough to achieve the desired result.
Over the years camerat manufacturers have developed power zoom lens for lens shutter cameras. This enable a photographer to quickly and smoothly change the focal length of the camera lens. Lens shutter caneras typically employ a triangulation technique to determine the distance of an object to be photographed from the camera. In the triangulation technique, a source of light, such as infared or ultrasonic, is emitted by the camera. The light source is bounced off of an object to photographed and returned to the camera. By utilizing the triangulation technique, the distance, "a" of the object can be determined.
The magnification value of a lens shutter camera is determined by the equation m=f/a, where "m" equals the magnfication value, "f" equals the focal length of the lens and "a" equals the distance of the objects from the camera. If a constant picture image size is to be maintained, the lens shutter camera must merely determine the distance of the object from the camera and change the focal lenght of the lens according to the above equation, so that the magnification value, "m", remains constant. This is easily accomplished with a power zoom, auto-focus, lens shutter camera.
Many individuals prefer using interchangeable lens camera, such as SLR cameras, because a plurality of different lenses can be easily attached to a camera body. It would be disireable to enable an interchangealbe lens camera to also be able to maintain a constant picture image size as the object to be photographed moves relative to the camera. However, constant image magnification techniques that are applicable to lens shutter cameras are not applicable to interchangealbe lens cameras.
In an interchangeable lens camera, triangulation techniques are generally not used for focusing purposes because of inherent inaccuracies. Such inaccuracies are acceptable when a small size lens (ie. 70 mm) is used, but is naccceptable when a large size lens (ie. 200 mm) is employed. Due to the intrinsic errors of the triangulation technique, manufacturers developed a thchnique for focusing an object based on the amount of defocus of the object to be photographed. Two defocus techniques are phase difference detection (PDD) method and contrast defference (CD) method.
A camera employing the defocus technique for focusing does not determined the distance, "a" of an object from the camera. Thus it is pot possible to maintain a constant imge magnification value, "m" as the distance, "a" of an object changes, using the techniques developed for lens shutter cameras (ie., m=f/a), because the typical interchangeable lens camera is to be able to maintain a constant image magnification value as the distance of an object from the camera moves, distance of an object to be photographed from the camera moves.
Two methods exist for making an image on a film frame occupy the same amount of space, even though the distance between the object to be photographed and the camera varies. In the furst method, the photographer perliminarily sets the size of the image before taking a picture. For instance, the photographer can select a portrait mode of operation, after which all pictures that are taken will be of teh portrait type. In the second method, the camera photographer likes the size of, the image (as shown in the viewfinder), the operator "fixes" the size. Thereafter, even if the object moves or the lens zooms. the image size of the object will be fixed. The present invention embodies both methods for making the image constant.
An image magnification control device for a camera, using a linking mechanism for a zoom lens device is disclosed, for example, in Japanese Patent Publication No. SH060-1602. In this prior art, a cam mechanism provided in a zoom lens controls the amount of zooming of the zoom lens so as to keep the ratio of a real subject length and a real focal length constant. Moreover, In this cam mechanism, a cam surface provided in a lens barrel is formed in a shape of a logarithmic curve. By touching a linked roller which travels in the forward and backward directions in parallel with the lens axis to the cam surface using a spring, as the lens barrel rotates, the linked roller follows the cam surface. In addition, a variable resistor is linked with the linked roller so as to control the amount of zooming by means of such a variable resistor.
However, it is preferred that the position where the linked roller touches the cam surface be at the intersection of a plane including the center line of the linked roller and the lens axis and of the cam surface.
However, there is a limit to decrease the radius of the linked roller. Since the cam surface is in the shape of a logarithmic curve rather than a linear shape, the position where the linked roller touches the cam surface deviates from the intersection described above due to a change of slope of the cam surface. Thus, there is a tendency where the amount of deviation depends on the degree of the slope of the cam surface. Such a deviation is undesirable for a precise zooming control.
In addition, it is very difficult to accurately machine a cam surface in the shape of a logarithmic curve with a high precision.
Further, a camera which is equipped with a CPU used for conducting an autofocus control and process control has been developed. For such type camera, motors therein may drive a zoom lens and conduct said image magnification.
When a release process is performed while a subject is out of the range where the image magnification can be controlled, a photograph is taken in the condition where a predetermined image magnification is not obtained resulting in undesirable situation.
In this case, when a subject is moved out of a range where the image magnification can be controlled, the image magnification control is temporarily-stopped When the subject is moved into the allowable range of the image magnification control, it is desirable to resume the image magnification control. In addition, it is preferred that this operation be conducted simply.
Moreover, there has been a camera which is equipped with an interchangeable lens. For such type camera, motors therein may drive a zoom lens and conduct said image magnification.
In the meantime, when such an image magnification control is performed in the sequence shot mode, if a subject is moved after it has been focused until the next release process is conducted, the next shots would be unfocused.
Further, new types of electronically controlled cameras have been developed such as autofocus control and program control. Conventionally, a camera has been known wherein a focus lock takes place when a subject is focused and a light metering operation is started by turning on a light metering switch in an auto and in-focus priority mode. As another prior art, a camera has been devised wherein pressing an image magnification mode setting switch allows an autofocus operation and power zoom operation to take place regardless of a distance to a subject so as to automatically control an image magnification of a photographic lens. By combining such functions, although it can be considered to enhance the performance of cameras, it is desirable to fully bring both the functions to the cameras.